I. What is the Clearance of a? Transformer
1. Professional Definition
Clearance:
The minimum straight-line air distance between two live conductors at different potentials, or between a live conductor and an earthed metal part.
It is measured directly through air, without following insulating surfaces, creepage paths, or relying on the thickness of insulating materials.
2. Simple Explanation
Insulating paper and insulating cylinders can degrade due to damage, aging, or moisture
exposure.
In contrast, air remains the most dependable insulating medium.
Clearance acts as the ultimate safety
margin: even if all solid insulation fails entirely, the air gap must still prevent electrical breakdown and arcing.
II. Clearance vs. Insulation Distance:
Many professionals in the industry often conflate these two concepts, which will now be clearly and definitively clarified:
Insulation Distance:
Encompasses all forms of insulation measures, including:
- Thickness of insulating paper
- Isolation provided by
insulating barrels
- Spacer-based isolation
- Creepage distance along surfaces
- Air clearance
Clearance:
Refers solely to the straight-line air distance. It is the most critical parameter within insulation distance, mandated by national standards and carrying veto authority.
Brief Summary:
Insulation distance constitutes a comprehensive system, whereas clearance represents the non-negotiable safety red line within that system.
III. Why Must Electrical Clearances TransformersBe Strictly Controlled?
Approximately
90% of on-site incidents-including air breakdowns, flashovers, transient discharges, and failed withstand-voltage tests-originate from the following root causes:
1. Excessively short separation between high-voltage and low-voltage
terminals
2. Inadequate clearance between leads and ground
3. Insufficient inter-phase air gaps
4. Reduced clearances due to assembly tolerances or deviations
5. Failure to apply altitude-based clearance derating (required above 1000 m)
While insulation thickness can be increased, air-gap dimensions must never be compromised.
Core Principles
1. Electrical clearance increases exponentially with voltage level.
2. Clearances must be adjusted upward at higher altitudes (correction required above 1000 m).
3. In environments with higher pollution severity, increasing clearance takes precedence over thickening solid insulation.
4. The required clearance for impulse withstand voltage exceeds that for power-frequency withstand voltage.
IV. Design Verification Methods for Electrical Clearances
1. End-part Clearance Verification (Most Critical)
Confirm the shortest straight-line distances from the terminals of high-voltage windings to ground, clamps, iron cores, and low-voltage winding ends.
2. Lead Clearance Verification
Assess air gaps between bare leads, copper busbars, and bushing bases relative to the tank, steel structures, and adjacent phases.
3. Cross-region Clearance Verification
Measure direct straight-line air distances at the outer peripheries of windings, air ducts, and insulation cylinders-without accounting for any insulating barriers.
Ironclad Design Rule
When verifying electrical clearances, the presence or thickness of any insulating material must be disregarded. All evaluations must be based exclusively on the shortest air-gap distance.
This is the most frequent error committed by approximately 90% of entry-level designers.
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